System and method for determining stroke volume of a patient

ABSTRACT

A PPG system for determining a stroke volume of a patient includes a PPG sensor configured to be secured to an anatomical portion of the patient. The PPG sensor is configured to sense a physiological characteristic of the patient. The PPG system may include a monitor operatively connected to the PPG sensor. The monitor receives a PPG signal from the PPG sensor. The monitor includes a pulse trending module determining a slope transit time of an upslope of a primary peak of the PPG signal. The pulse trending module determines a stroke volume of the patient as a function of the slope transit time.

This application is a continuation of U.S. patent application Ser. No.14/977,301, which was filed on Dec. 21, 2015, is entitled, “SYSTEM ANDMETHOD FOR DETERMINING STROKE VOLUME OF PATIENT,” and issued as U.S.Pat. No. 10,448,851 on Oct. 22, 2019 and is a divisional of U.S. patentapplication Ser. No. 13/609,566, which was filed on Sep. 11, 2012, isentitled, “SYSTEM AND METHOD FOR DETERMINING STROKE VOLUME OF PATIENT,”and issued as U.S. Pat. No. 9,241,646 on Jan. 26, 2016. The entirecontent of each of U.S. patent application Ser. No. 14/977,301 and U.S.patent application Ser. No. 13/609,566 is incorporated herein byreference.

BACKGROUND

Embodiments of the present disclosure generally relate to physiologicalsignal processing and, more particularly, to processing physiologicalsignals to determine a stroke volume or cardiac output of a patient. Incardiovascular physiology, stroke volume (SV) is the volume of bloodpumped from one ventricle of the heart with each beat. SV is calculatedusing measurements of ventricle volumes from an echocardiogram (ECG) andsubtracting the volume of the blood in the ventricle at the end of abeat (end-systolic volume) from the volume of blood just prior to thebeat (called end-diastolic volume). Stroke volume is an importantdeterminant of cardiac output, which is the product of stroke volume andheart rate. Because stroke volume decreases in certain conditions anddisease states, stroke volume itself correlates with cardiac function.

Systems and methods have been used to measure stroke volume. Forexample, photoplethysmogram (PPG) systems have been used in connectionwith ECG systems to aid in determining SV by measuring pulse transittimes from the pulse measurement by the ECG system and the pulsemeasurement by the PPG system. The PPG system performs a non-invasive,optical measurement that may be used to detect changes in blood volumewithin tissue, such as skin, of an individual.

SUMMARY

Certain embodiments provide a PPG system for determining a stroke volumeof a patient. The PPG system may include a PPG sensor configured to besecured to an anatomical portion of the patient. The PPG sensor isconfigured to sense a physiological characteristic of the patient. ThePPG system may include a monitor operatively connected to the PPGsensor. The monitor receives a PPG signal from the PPG sensor. Themonitor includes a pulse trending module configured to determine a slopetransit time of an upslope of a primary peak of the PPG signal. Thepulse trending module configured to determine a stroke volume of thepatient as a function of the slops transit time. The PPG system mayinclude a monitor that displays an output of the stroke volume on adisplay.

Optionally, the slope transit time may be calculated based upon a unitamplitude of the pulse. The slope transit time may be calculated by thepulse trending module as a temporal gradient of the PPG signal. Theslope transit time may be calculated by the pulse trending module as aninverse of a gradient of the upslope of the primary peak of the PPGsignal.

The PPG signal may form a PPG waveform. The pulse trending module mayanalyze a contour of the PPG waveform along the upslope of the primarypeak to identify a gradient of the contour per unit amplitude. The pulsetrending module may calculate the slope transit time as the inverse ofthe gradient. The pulse trending module may analyze a contour of the PPGwaveform along the upslope of the primary peak to identify a temporalgradient of the contour per unit amplitude. The pulse trending modulemay calculate the slope transit time as a function of the temporalgradient.

The pulse trending module may analyze a contour of the PPG waveformalong the upslope of the primary peak to identify at least one waveformcharacteristic thereof. The waveform characteristic may include at leastone of an amplitude (A) of the contour, a gradient (m) of the contourand a temporal gradient (k) of the contour. The pulse trending modulemay calculate the stroke volume based upon at least one of the ratios ofm=A/STT, k=1/m and k=STT.

The pulse trending module may calculate stroke volume based uponempirically determined constants. The pulse trending module may use acalibration constant to determine the stroke volume.

Certain embodiments provide a PPG system for determining a stroke volumeof a patient. The PPG system may Include a PPG sensor configured to besecured to an anatomical portion of the patient to sense a physiologicalcharacteristic of the patient and generate a PPG signal. The PPG signalmay have a primary peak, a trailing peak and a dicrotic notch for eachpulse. The PPG system may include a monitor operatively connected to thePPG sensor. The monitor receives the PPG signal from the PPG sensor. Themonitor includes a pulse trending module configured to determinewaveform characteristics of an upslope of the primary peak of the PPGsignal. The waveform characteristics may include an amplitude a gradientand a slope transit time of the PPG signal. The pulse trending modulemay be configured to determine a stroke volume as a function of thewaveform characteristics of the upslope of the primary peak of the PPGsignal.

Certain embodiments provide a method of determining a stroke volume of apatient from a PPG system. The method may include securing a PPG sensorto an anatomical portion of the patient, sensing a physiologicalcharacteristic of the patient with the PPG sensor and receiving a PPGsignal from the sensor at a monitor. The monitor includes a pulsetrending module. The method includes analyzing the PPG signal at thepulse trending module to determine a slope transit time of an upslope ofa primary peak of the PPG signal. The method includes calculating astroke volume at the pulse trending module of the patient based on theslope transit time of the upslope of the primary peak of the PPG signal.

The analyzing operation may include correlating the slope transit timeof the upslope of the primary peak of the PPG signal to a slope transittime of an initial pressure wave. The analyzing operation may includeanalyzing a contour of the PPG waveform along the upslope of the primarypeak to identify at least one waveform characteristic thereof, thewaveform characteristic including at least one of an amplitude (A) ofthe contour, a gradient (m) of the contour and a temporal gradient (k)of the contour. The pulse trending module may calculate the slopetransit time based upon at least one of the ratios of A/STT, k=1/m andk=STT.

The correlating operation may include calculating the slope transit timeof the initial pressure wave as a function of effects of reflected wavesof previous pulses based on at least one of wave periods, pulse periods,pulse transit times, and shapes of the previous waveforms.

The calculating operation may include calculating the stroke volumebased upon the slope transit time and empirically determined constants.

Certain embodiments provide a tangible and non-transitory computerreadable medium that includes one or more sets of instructionsconfigured to direct a computer to receive a physiological signal from asensor secured to an anatomical portion of a patient, determine a slopetransit time of an upslope of a primary peek of the PPG signal, andcalculate e stroke volume of the patient based on the slope transit timeof the upslope of the primary peak of the PPG signal.

Embodiments of the present disclosure allow for quick and simpledetermination of stroke volume through analysis of a PPG signal. The PPGsignal may be obtained from a single pleth-only system. In contrast toprevious systems and methods, embodiments may not require multiple PPGsensors at multiple locations on the patient to determine pulse transittimes for calculating stroke volume, in contrast to previous systems andmethods, embodiments may not require another type of monitoring systemsuch as an ECG system for measuring physiological conditions of thepatient for use in calculations for the stroke volume. Moreover,embodiments may be used to measure temporal components of the PPGwaveform that are independent of pulse transit times, respiratoryeffects and the like.

Certain embodiments may include some, all, or none of the aboveadvantages. One or more other technical advantages may be readilyapparent to those skilled in the art from the figures, descriptions, andclaims included herein. Moreover, while specific advantages have beenenumerated above, various embodiments may include all, some, or none ofthe enumerated advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a simplified block diagram of a system configured todetermine a physiological parameter of a patient, according to anembodiment.

FIG. 1B illustrates an electrocardiogram (EGG) waveform of the patient,according to an embodiment.

FIG. 1C illustrates a phonocardiogram (PCG) waveform of the patient,according to an embodiment.

FIG. 1D illustrates a photoplethysmogram (PPG) waveform of the patient,according to an embodiment.

FIG. 2 illustrates an isometric view of a PPG system, according to anembodiment.

FIG. 3 illustrates a simplified block diagram of a PPG system, accordingto an embodiment.

FIG. 4 is an illustrative processing system in accordance with anembodiment.

FIG. 6 illustrates a PPG signal over time, according to an embodiment.

FIG. 6 illustrates a pulse waveform of a PPG signal, according to anembodiment.

FIG. 7 illustrates a pulse waveform of a PPG signal according to anembodiment.

FIG. 8 illustrates a flow chart of a method of determining a strokevolume of a patient, according to an embodiment.

FIG. 9 illustrates a flow chart of a method of operating a PPG system,according to an embodiment.

DETAILED DESCRIPTION

FIG. 1A illustrates a simplified block diagram of a system 100configured to determine a physiological parameter of a patient 102. Thesystem 100 is configured to acquire physiological signals (orbiosignals) from the patient 102 and analyze the physiological signalsto determine a physiological parameter, such as a stroke volume (SV)and/or a cardiac output (CO) of the patient 102. The SV is the volume ofblood leaving the heart in a given contraction. The CO is the volume ofblood being pumped by the heart, and is a function of SV and a heartrate (HR) of the patient 102. The physiological signals are indicativeof phenomena occurring in the patient. For example, the physiologicalsignals may describe cardiac activity in which the heart undergoes anumber of cardiac cycles (e.g., heart beats). The physiological signalsmay be electrical, optical, and/or acoustical signals.

The system 100 may include a sensor 104 that is configured to detect oneor more types of physiological signals. By way of example, the system100 may include an electrocardiogram (BCG) system that detectselectrical signals corresponding to muscle excitation of the heart. Insuch cases, the sensor 104 may include a plurality of electrodes thatare coupled to different anatomical locations of the patient 102 (e.g.,chest, wrists, and/or erodes). FIG. 1B illustrates, according to anembodiment, a representative ECO waveform 110A based on the ECG signalsacquired by the electrode-sensor 104.

As another example, the system 100 may include a phonocardiogram (PCG)system that detects sounds that may be caused by the closing of heartvalves. In such cases, the sensor 104 may include one or moremicrophones that are coupled to the patient 102. FIG. 1C illustrates,according to an embodiment, a representative PCG waveform 110B based onthe PCG signals acquired by the microphone-sensor 104. In alternativeembodiments, the system 100 includes an ultrasound system configured todetect heart beats from the patient 102.

In certain embodiments, the system 100 includes a photoplethysmogram(PPG) system, which can measure changes in blood volume through ananatomical portion or location (e.g., a finger). A typical example of aPPG system is a pulse oximetry system although other PPG systems existand may be used with embodiments described herein. The PPG-sensor 104may include a probe having one or more light sources and one or morelight detectors that are coupled to the patient 102. The fight sources)provide an incident light that is scattered, absorbed, reflected, and/ortransmitted by the blood. The light detectors) detect an amount of lightthat may correspond to blood volume. For example, as the volume of bloodincreases at the anatomical location, the light is attenuated more and,as such, a smaller amount of light is detected. FIG. 1C illustrates,according to an embodiment, a representative PPG waveform 110C based onthe PPG signals acquired by the PPG-sensor 104.

As shown in FIG. 1A, the system 100 may include a monitor (or computingsystem) 106 that includes one or more components for analyzing and/orprocessing the physiological signals. For example, the monitor 106 mayinclude a pre-processing module 112, a validation module 113, arate-determining module 114, an analysis module 115, and a graphicaluser interface (GUI) module 116. As used herein, a “module” may includehardware components (e.g., processor, controller), software components,or a combination thereof including any associated circuitry.

The pre-processing module 112 is configured to remove unwanted signaldate (e.g., noise) from raw physiological signal data obtained from theindividual 102. For example, raw PPG signals may include artifactscaused by motion of the patient relative to the light detector,instrumentation bias (e.g., bias by amplifiers used in the PPG system),powerline interference, low amplitude PPG signals, etc. Rawphysiological signals from other types of monitoring systems, such asECG and PCG systems, may also include unwanted noise. The pre-processingmodule 112 is configured to remove the noise to provide clearer and/orcleaner physiological signals to the other components of the system 100.

The validation module 113 is configured to analyze the physiologicalsignals to identify valid heart beats and waveforms from thephysiological signals. In some embodiments, the validation module 113 ispart of the pre-processing module 112 or another module. The validationmodule 113 may analyze the physiological signals after the physiologicalsignals have been processed. In some embodiments, the validation module113 examines the physiological signals to identify one or more referencefeatures in the physiological signals. For instance, a series of datapoints over time may provide waveforms, such as the waveforms 110A-110C.A reference feature may be an identifiable point, segment, orcharacteristic of the waveform (e.g., peak, trough (or foot), notch,slope of a designated segment threshold, etc.) that may be relied uponin analysis of the physiological signals. In many cases, a referencefeature of a waveform corresponds to a known physiological activity(e.g., excitation of heart muscles, closure or opening of valves,maximum volume of blood at an anatomical location, etc.). The validationmodule 113 may examine the data points, or a select number of datapoints (e.g., a segment of the waveform), to confirm that the datapoints are caused by a designated event of a cardiac cycle and are not aresult of noise or other unwanted event, such as when the sensor 104 isbeing adjusted. The data points associated with valid heart beats maythen be used by a rate-determining module 114 to determine a heart ratesignal. In some embodiments, the data points that are not identified ascorresponding to heart beats may not be considered in subsequentanalysis.

The rate-determining module 114 is configured to analyze the heart beatsor, more specifically, the data points corresponding to the valid heartbeats identified by the validation module 113 and determine a HR of theindividual at a designated moment of time. For example, the HR may becalculated by analyzing time intervals between two of more heart beatsor by analyzing portions of a waveform that corresponds to a singleheart beat. By way of example only, when analyzing the physiologicalsignals, the rate-determining module 114 may identify one or morereference features (e.g., points, segments, and/or characteristics thatcorrespond to a waveform) that may be used to calculate HR. For example,in the EGG waveform 110A, the rate-determining module 114 may identifyan R-wave peak 118 in each heart beat. A time interval 120 between theR-wave peaks 118A, 118B may be determined and divided by a unit of timeto calculate the HR. For example, if the time interval is 0.90 secondsbetween the two R-wave peaks 118A, 118B, then the HR is 67 beats/minute.

Corresponding to each heart beat, the PPG waveform 1100 may include asystolic peak 122, a diastolic peak 124, and a dicrotic notch 126 thatexists therebetween. In some cases, the diastolic peak is not a peak butinstead a change in slope. To determine HR, the rate-determining module114 may identify for each heart beat a reference point that exists at afoot 128 of the wave before the systolic peak 122. The HR may bedetermined in a similar manner as described above with respect to theEGG waveform by identifying a time interval 129 between the foot 128Aand the foot 128B.

However, it should be noted that the above description is just exemplaryand that many reference points and/or waveform segments may be analyzedand used to calculating a HR of an individual. Furthermore, thephysiological signals may be processed in various manners to determine aHR. For example, a first derivative or second derivative of the PPGwaveform may be used to locate certain reference data points m the PPGwaveform. Such reference data points may be used for determining theheart rate or for determining other physiological parameters.

As will be described in greater detail below, the analysis module 115 isconfigured to identify data points from the physiological signals. Thesignal data points may be a limited number of data points from a seriesof data points. For example, the signal data points may correspond to apeak data point, a trough data point, a segment of data points thatcorrespond to a slope of the waveform, and the like. To calculate aphysiological parameter, such as the SV and/or CO, the analysis module115 may use one or more of the data points to calculate thephysiological parameter.

The system 100 may also include a user interface 130 that includes adisplay 132. The user interface 130 may include hardware, firmware,software, or a combination thereof that enables a user to directly orindirectly control operation of the system 100 and the variouscomponents thereof. The display 132 is configured to display one or moreimages, such as one or more of the waveforms 110A-110C. The display 132may also be configured to show a representation of the physiologicalparameter, for example, a number representing the SV or the CO. In someembodiments, the user interface 130 may also include one or more inputdevices (not shown), such as a physical keyboard, mouse, touchpad,end/or touch-sensitive display. The user interface 130 may beoperatively connected to the GUI module 116 and receive instructionsfrom the GUI module 116 to display designated images on the display 132.The user interface 130 may also include a printer or other device forproviding (e.g. printing) a report. The user interface 130 may alsoinclude an alarm or alert system.

There are many medical conditions in which SV and CO are relevant. Forexample, uncontrolled atrial fibrillation is a condition characterizedby an abnormally rapid heart rate caused by unregulated firing ofelectrical pulses within the heart muscles. This rapid firing induces anelevated heart rate (known as tachycardia) while simultaneouslypreventing the ventricles from filling completely with blood before thenext contraction. In this condition, the HR increases while SVdecreases. As a result the CO (total volume of blood pumped to the bodyfrom the heart) can decrease during atrial fibrillation, leading to adecrease in blood pressure (BP).

Detecting a change in the SV, CO, HR and/or BP can alert medicalproviders to potentially dangerous patient conditions. Analyzing and/orprocessing the physiological signals to provide a representation of SVand/or CO on a display for a care provider may be more meaningful thanmerely monitoring BP and HR readings. A monitoring system that tracksand provides information relating to SV and/or CO for a care providerand indicates a patient status in response to calculated SV and/or COprovides a tool in patient diagnosis and treatment. The presentdisclosure relates to systems and methods for detecting SV and/or CO todetermine patient status, and more particularly, relates to analyzing atrending nature of a waveform to determine SV and/or CO to alert a careprovider to a patient condition. For example, the present disclosurerelates to systems and methods that analyze a slope transit time of aPPG signal for determining SV and/or CO.

FIG. 2 illustrates an isometric view of a PPG system 210, according toan embodiment. The PPG system 210 may be used as part of the system 100(shown in FIG. 1). While the system 210 is shown and described as a PPGsystem 210, the system may be various other types of physiologicaldetection systems, such as an electrocardiogram system, aphonocardiogram system, and the like. The PPG system 210 may be a pulseoximetry system, for example. The system 210 may include a PPG sensor212 and a PPG monitor 214. The PPG sensor 212 may include an emitter 216configured to emit light into tissue of a patient. For example, theemitter 216 may be configured to emit light at two or more wavelengthsinto foe tissue of the patient. The PPG sensor 212 may also include adetector 218 that is configured to detect the emitted light from theemitter 216 that emanates from the tissue after passing through thetissue.

The system 210 may include a plurality of sensors forming a sensor arrayin place of the PPG sensor 212. Each of the sensors of the sensor arraymay be a complementary metal oxide semiconductor (CMOS) sensor, forexample. Alternatively, each sensor of the array may be a chargedcoupled device (CCD) sensor, in another embodiment, the sensor array mayinclude a combination of CMOS and CCD sensors. The CCD sensor mayinclude a photoactive region and a transmission region configured toreceive and transmit, while the CMOS sensor may include an integratedcircuit having an array of pixel sensors. Each pixel may include aphotodetector and an active amplifier.

The emitter 216 and the detector 218 may be configured to be located atopposite sides of a digit, such as a finger or toe, in which case thelight that is emanating from the tissue passes completely through thedigit. The emitter 216 and the detector 218 may be arranged so thatlight from the emitter 216 penetrates the tissue and is reflected by thetissue into the detector 218, such as a sensor designed to obtain pulseoximetry data.

The sensor 212 or sensor array may be operatively connected to and drawpower from the monitor 214. Optionally, the sensor 212 may be wirelesslyconnected to the monitor 214 and include a battery or similar powersupply (not shown). The monitor 214 may be configured to calculatephysiological parameters based at least in part on data received fromthe sensor 212 relating to light emission and detection. Alternatively,the calculations may be performed by and within the sensor 212 and theresult of the oximetry reading may be passed to the monitor 214.Additionally, the monitor 214 may include a display 220 configured todisplay the physiological parameters or other information about thesystem 210. The monitor 214 may also include a speaker 222 configured toprovide an audible sound that may be used in various other embodiments,such as for example, sounding an audible alarm in the event thatphysiological parameters are outside a predefined normal range.

The sensor 212, or the sensor array, may be communicatively coupled tothe monitor 214 via a cable 224. Alternatively, a wireless transmissiondevice (not shown) or the like may be used instead of, or In additionto, the cable 224.

The system 210 may also include a multi-parameter workstation 226operatively connected to the monitor 214. The workstation 226 may be orinclude a computing sub-system 230, such as standard computer hardware.The computing sub-system 230 may include one or more modules and controlunite, such as processing devices that may include one or moremicroprocessors, microcontrollers, integrated circuits, memory, such asread-only and/or random access memory, and the like. The workstation 226may include a display 228, such as a cathode ray tube display, a flatpanel display, such as a liquid crystal display (LCD), light-emittingdiode (LED) display, a plasma display, or any other type of monitor. Thecomputing sub-system 230 of the workstation 226 may be configured tocalculate physiological parameters and to show information from themonitor 214 and from other medical monitoring devices or systems (notshown) on the display 228. For example, the workstation 226 may beconfigured to display SV information, CO information, an estimate of apatient's blood oxygen saturation generated by the monitor 214 (referredto as an SpO2 measurement), pulse rate information from the monitor 214and blood pressure from a blood pressure monitor (not shown) on thedisplay 228.

The monitor 214 may be communicatively coupled to the workstation 226via a cable 232 and/or 234 that is coupled to a sensor input port or adigital communications port, respectively and/or may communicatewirelessly with the workstation 226. Alternatively, the monitor 214 andthe workstation 226 may be integrated as part of a common device.Additionally, the monitor 214 and/or workstation 228 may be coupled to anetwork to enable the sharing of information with servers or otherworkstations. The monitor 214 may be powered by a battery or by aconventional power source such as a wall outlet.

FIG. 3 frustrates a simplified block diagram of the PPG system 210,according to an embodiment, when the PPG system 210 is a pulse oximetrysystem, the emitter 216 may be configured to emit at least twowavelengths of light (for example, red and infrared) into tissue 240 ofa patient. Accordingly, the emitter 216 may include a red light-emittinglight source such as a red light-emitting diode (LED) 244 and aninfrared light-emitting light source such as an infrared LED 246 foremitting light into the tissue 240 at the wavelengths used to calculatethe patient's physiological parameters. For example, the red wavelengthmay be between about 600 nm and about 700 nm, and the infraredwavelength may be between about 800 nm and about 1000 nm. In embodimentswhere a sensor array is used in place of single sensor, each sensor maybe configured to emit a single wavelength. For example, a first sensormay emit a red light while a second sensor may emit an infrared light.

As discussed above, the PPG system 210 is described m terms of a pulseoximetry system. However, the PPG system 210 may be various other typesof systems. For example, the PPG system 210 may be configured to emitmore or less than two wavelengths of light into the tissue 240 of thepatient. Further, the PPG system 210 may be configured to emitwavelengths of light other than red and infrared into the tissue 240. Asused herein, the term “light” may refer to energy produced by radiativesources and may include one or more of ultrasound, radio, microwave,millimeter wave, infrared, visible, ultraviolet, gamma ray or X-rayelectromagnetic radiation. The light may also include any wavelengthwithin the radio, microwave, infrared, visible, ultraviolet or X-rayspectra, and that any suitable wavelength of electromagnetic radiationmay be used with the system 210. The detector 218 may be configured tobe specifically sensitive to the chosen targeted energy spectrum of theemitter 216.

The detector 218 may be configured to detect the intensity of light atthe red and infrared wavelengths. Alternatively, each sensor in thearray may be configured to detect an intensity of a single wavelength.In operation, light may enter the detector 218 after passing through thetissue 240. The detector 218 may convert the intensity of the receivedlight into an electrical signal. The light intensity may be directlyrelated to the absorbance and/or reflectance of light in the tissue 240.For example, when more light at a certain wavelength is absorbed orreflected, less light of that wavelength is received from the tissue bythe detector 218. After converting the received light to an electricalsignal, the detector 218 may send the signal to the monitor 214, whichcalculates physiological parameters based on the absorption of the redand infrared wavelengths in the tissue 240.

In an embodiment, an encoder 242 may store information about the sensor212, such as sensor type (for example, whether the sensor is intendedfor placement on a forehead or digit) and the wavelengths of lightemitted by the emitter 216. The stored information may be used by themonitor 214 to select appropriate algorithms, lookup tables and/orcalibration coefficients stored in the monitor 214 for calculatingphysiological parameters of a patient. The encoder 242 may store orotherwise contain information specific to a patient, such as, forexample, the patient's age, weight, and diagnosis. The information mayallow the monitor 214 to determine, for example, patient-specificthreshold ranges related to the patient's physiological parametermeasurements, and to enable or disable additional physiologicalparameter algorithms. The encoder 242 may, for instance, be a codedresistor that stores values corresponding to the type of sensor 212 orthe types of each sensor to the sensor array, the wavelengths of lightemitted by emitter 216 on each sensor of the sensor array, and/or thepatient's characteristics. Optionally, the encoder 242 may include amemory in which one or more of the following may be stored forcommunication to the monitor 214; the type of the sensor 212, thewavelengths of light emitted by emitter 216, the particular wavelengtheach sensor in the sensor array is monitoring, a signal threshold foreach sensor to the sensor array, any other suitable information, or anycombination thereof.

Signals from the detector 218 and the encoder 242 may be transmitted tothe monitor 214. The monitor 214 may include a general-purpose controlunit, such as a microprocessor 248 connected to an internal bus 250. Themicroprocessor 248 may be configured to execute software, which mayinclude an operating system and one or more applications, as part ofperforming the functions described herein. A read-only memory (ROM) 252,a random access memory (RAM) 254, user inputs 256, the display 220, andthe speaker 222 may also be operatively connected to the bus 250. Thecontrol unit and/or the microprocessor 248 may include a pulse trendingmodule 249 that is configured to determine a trending nature, index orvalue of the PPG signals or waveform. In an embodiment, the pulsetrending module 249 analyzes the PPG signal to determine a slope transittime of a segment of the PPG signal as a basis for determining thestroke volume of the patient. The pulse trending module 249 isconfigured to determine a stroke volume based on calculations,measurements or other information, data or signals received from the PPGsensor 212.

The RAM 254 and the ROM 252 are illustrated by way of example, and notlimitation. Any suitable computer-readable media may be used in thesystem for data storage. Computer-readable media are configured to storeinformation that may be interpreted by the microprocessor 248. Theinformation may be data or may take the form of computer-executableinstructions, such as software applications, that cause themicroprocessor to perform certain functions and/or computer-implementedmethods. The computer-readable media may include computer storage mediaand communication media. The computer storage media may include volatileand non-volatile media, removable and non-removable media implemented inany method or technology for storage of information such ascomputer-readable instructions, data structures, program modules orother data. The computer storage media may include, but are not limitedto, RAM, ROM, EPROM EEPROM, flash memory or other solid state memorytechnology, CD-ROM, DVD, or other optical storage, magnetic cassettes,magnetic tape, magnetic disk storage or other magnetic storage devices,or any other medium which may be used to store desired information andthat may be accessed by components of the system.

The monitor 214 may also include a time processing unit (TPU) 258configured to provide timing control signals to a light drive circuitry260, which may control when the emitter 216 is illuminated andmultiplexed timing for the red LED 244 and the infrared LED 246. The TPU258 may also control the gating-in of signals from the detector 218through an amplifier 262 and a switching circuit 264. The signals aresampled at the proper time, depending upon which light source isilluminated. The received signal from the detector 218 may be passedthrough an amplifier 266, a tow pass filter 268, and ananalog-to-digital converter 270. The digital data may then be stored ina queued serial module (QSM) 272 (or buffer) for later downloading toRAM 254 as GSM 272 fills up. In an embodiment, there may be multipleseparate parallel paths having amplifier 266, filter 268, and A/Dconverter 270 for multiple light wavelengths or spectra received.

The microprocessor 248 may be configured to determine the patient'sphysiological parameters, such as SV, CO, SpO2, pulse rate, and thelike, using various algorithms and/or look-up tables based on thevalues) of the received signals and/or data corresponding to the lightreceived by the detector 218. The signals corresponding to informationabout a patient, and regarding the intensity of light emanating from thetissue 240 over time, may be transmitted from the encoder 242 to adecoder 274. The transmitted signals may include, for example, encodedinformation relating to patient characteristics. The decoder 274 maytranslate the signals to enable the microprocessor 248 to determine thethresholds based on algorithms or look-up tables stored in the ROM 252.The user inputs 256 may be used to enter information about the patient,such as age, weight, height, diagnosis, medications, treatments, and soforth. The display 220 may show a list of values that may generallyapply to the patient, such as, for example, age ranges or medicationfamilies, which the user may select using the user inputs 266.

As noted, the PPG system 210 may be a pulse oximetry system. A pulseoximeter is a medical device that may determine oxygen saturation ofblood. The pulse oximeter may indirectly measure the oxygen saturationof a patient's blood (as opposed to measuring oxygen saturation directlyby analyzing a blood sample taken from the patient) and changes in bloodvolume in the skin. Ancillary to the blood oxygen saturationmeasurement, pulse oximeters may also be used to measure the pulse rateof a patient. Pulse oximeters typically measure and display variousblood flow characteristics including, but not limited to, the oxygensaturation of hemoglobin in arterial blood.

A pulse oximeter may include a light sensor, similar to the sensor 212,that is placed at a site on a patient, typically a fingertip, toe,forehead or earlobe, or in the case of a neonate, across a foot. Thepulse oximeter may pass light using a light source through bloodperfused tissue and photoelectrically sense the absorption of light inthe tissue. For example, the pulse oximeter may measure the intensity oflight that is received at the light sensor as a function of time. Asignal representing light intensity versus time or a mathematicalmanipulation of this signal (for example, a scaled version thereof, alog taken thereof, a scaled version of a log taken thereof, and/or thelike) may be referred to m the photoplethysmogram (PPG) signal. Inaddition, the term “PPG signal” m used herein, may also refer to anabsorption signal (for example, representing the amount of lightabsorbed by the tissue) or any suitable mathematical manipulationthereof. The light intensity or the amount of light absorbed may then beused to calculate the slope transit time of the pulse, the amount of theblood constituent (for example, oxyhemoglobin) being measured as well asthe pulse rate and when each individual pulse occurs.

The light passed through the tissue is selected to be of one or morewavelengths that are absorbed by the blood in an amount representativeof the amount of the blood constituent present in the blood. The amountof light passed through the tissue varies in accordance with thechanging amount of blood constituent in the tissue and the related tightabsorption. Red and infrared wavelengths may be used because it has beenobserved that highly oxygenated blood will absorb relatively less redfight and more infrared light than blood with lower oxygen saturation.By comparing the intensities of two wavelengths at different points inthe pulse cycle, it is possible to estimate the blood oxygen saturationof hemoglobin in arterial blood.

Stroke volume may correlate with cardiac health and other physiologicalparameters important in patient care, such as fluid responsiveness.Fluid responsiveness relates to the volume of fluid, such as blood, inthe arteries, veins, and vasculature of an individual. In general, fluidresponsiveness may include a measurement of the response of strokevolume, the volume of blood passing out of the heart with eachheartbeat, to venous return, the volume of blood entering the heart witheach heartbeat, caused by the clinical administration of fluid into thevasculature, such as through an intravenous inaction. With eachheartbeat, a certain amount of blood is pumped out of the heart. Themore blood that fills the heart, the more blood the heart can pump outwith each heartbeat. If blood volume is too low, the heart may not fullyfill with blood. Therefore, the heart may not pump out as much bloodwith each heartbeat. Consequently, low blood volume may load to towblood pressure, and organs and tissues may not receive enough blood tooptimally and/or properly function. Monitoring stroke volume and fluidresponsiveness allows a physician to determine whether additional fluidshould be provided to a patient, such as through an intravenous fluidinjection. In short, fluid responsiveness represents a prediction ofwhether or not additional intravenous fluid may improve blood flowwithin a patient. Fluid responsiveness may be viewed as a response of aheart in relation to overall fluid within a patient.

Stroke volume may be monitored in, for example, critically-ill patientsbecause fluid administration plays an important role in optimizingcardiac output and oxygen delivery to organs and tissues. However,clinicians need to balance central blood volume depletion and volumeoverloading. Critically-ill patients are generally at greater risk forvolume depletion and severe hypotension is a common life-threateningcondition in critically-ill patients. Conversely, administering too muchfluid may induce life-threatening adverse effects, such as volumeoverload, systemic and pulmonary edema, and increased tissue hypoxia.Therefore, obtaining reliable information and parameters that aidclinicians in fluid management decisions may help improve patientoutcomes.

FIG. 4 is an illustrative processing system 400 in accordance with anembodiment. In an embodiment, an input signal generator 410 generates aninput signal 416. The input signal generator 410 includes apre-processor 420 coupled to a sensing device 418. It will be understoodthat the input signal generator 410 may include any suitable signalsource, signal generating data, signal generating equipment, or anycombination thereof to produce the signal 416. The signal 416 may be asingle signal, or may be multiple signals transmitted over a singlepathway or multiple pathways.

The pre-processor 420 may apply one or more signal processing techniquesto the signal generated by the sensing device 418. For example, thepre-processor 420 may apply a pre-determined transformation to thesignal provided by the sensing device 418 to produce an input signal 416that can be appropriately interpreted by the processor 412. Thepre-processor 420 may also perform any of the following operations tothe signal provided by the sensing device 418; reshaping the signal fortransmission; multiplexing the signal; modulating the signal ontocarrier signals; compressing the signal; encoding the signal; andfiltering the signal.

In the embodiment of FIG. 4, the signal 416 is coupled to the processor412. The processor 412 may be any suitable software, firmware, and/orhardware, and/or combinations thereof for processing the signal 416. Forexample, the processor 412 may include one or more hardware processors(e.g., integrated circuits), one or more software modules,computer-readable media such as memory, firmware, or any combinationthereof. The processor 412 may, for example, be a computer or may be oneor more chips (i.e., Integrated circuits). The processor 412 may, forexample, be configured of analog electronic components. The processor412 may perform some or all of the calculations associated with themonitoring methods of the present disclosure. For example, the processor412 may analyze the physiological signals, waveforms, and the like andcompute pulse trending characteristics thereof to determine a strokevolume and/or cardiac output, as discussed further below. The processor412 may also perform any suitable signal processing to filter the signal418, such as any suitable band-pass filtering, adaptive filtering,closed-loop filtering, and/or any other suitable filtering, and/or anycombination thereof. The processor 412 may also receive input signalsfrom additional sources (not shown). For example, the processor 412 mayreceive an input signal containing information about the patient ortreatments provided to the patient. These additional input signals maybe used by the processor 412 in any of the calculations or operations itperforms in accordance with the processing system 400.

The processor 412 may be coupled to one or more memory devices (notshown) or incorporate one or more memory devices such as any suitablevolatile memory device (e.g., RAM, registers, etc.), non-volatile memorydevice (e.g., ROM, EPROM, magnetic storage device, optical storagedevice, flash memory, etc.), or both, in an embodiment, the processor412 may store physiological measurements or previously received datafrom the signal 416 in a memory device for later retrieval. Theprocessor 412 may be coupled to a calibration device (not shown) thatmay generate or receive as input reference measurements for use incalibrating calculations.

The processor 412 is coupled to an output 414 through a patient statusindicator signal 419, and may be coupled through additional signalpathways not shown. The output 414 may be any suitable output devicesuch as, for example, one or more medical devices (e.g., a medicalmonitor that displays various physiological parameters, a medical alarm,or any other suitable medical device that either displays physiologicalparameters or uses the output of the processor 412 as an input), one ormore display devices (e.g., monitor, PDA, mobile phone, any othersuitable display device, or any combination thereof), one or more audiodevices, one or more memory devices (e.g., hard disk drive, flashmemory, RAM, optical disk, any other suitable memory device, or anycombination thereof), one or more printing devices, any other suitableoutput device, or any combination thereof. In an embodiment, the patientstatus indicator signal 419 induces at least one of an identification ofa medical condition of the patient; an alert; a current stroke volumemeasurement; a current cardiac output measurement; a current HRmeasurement; a current BP measurement; another current physiologicalmeasurement; an estimated patient status; and an estimated patientoutcome. In some embodiments, the patient status indicator signal 419will be stored in a memory device or recorded m another physical formfor future, further analysis.

It will be understood that the system 400 may be incorporated into thesystem 100 (shown in FIG. 1) and/or the system 210 (shown in FIGS. 2 and3) in which, for example, the input signal generator 410 may beimplemented as parts of the sensor 212 and/or the monitor 214 and theprocessor 412 may be implemented as part of the monitor 214, in someembodiments, portions of the system 400 may be configured to beportable. For example, all or a part of the system 400 may be, embeddedin a small, compact object carried with or attached to the patient(e.g., a watch, other piece of jewelry, or cellular telephone). In suchembodiments, a wireless transceiver (not shown) may also be included inthe system 400 to enable wireless communication with other components ofsystem 210. As such, the system 210 may be part of a fully portable andcontinuous monitoring solution.

FIG. 5 illustrates a PPG signal 600 over time, according to anembodiment. The PPG signal 500 is an example of a physiological signal.However, embodiments may be used in relation to various otherphysiological signals, such as electrocardiogram signals,phonocardiogram signals, ultrasound signals, and the like. The PPGsignal 500 may be determined, formed, and displayed as a waveform by themonitor 214 (shown in FIG. 2) that receives signal data from the PPGsensor 212 (shown in FIG. 2). For example, the monitor 214 may receivesignals from the PPG sensor 212 positioned on a finger of a patient. Themonitor 214 processes the received signals, and displays the resultingPPG signal 600 on the display 228 (shown in FIG. 2).

The PPG signal 500 may include a plurality of pulses 502 a-502 n over apredetermined time period. The time period may be a fixed time period,or the time period may be variable. Moreover, the time period may be arolling time period, such as a 5 second rolling timeframe.

Each pulse 502 a-502 n may represent a single heartbeat and may includea pulse-transmitted or primary peak 504 separated from a pulse-reflectedor trailing peak 506 by a dicrotic notch 508. The primary peak 504represents a pressure wave generated from the heart to the point ofdetection, such as in a finger where the PPG sensor 212 is positioned.The trailing peak 506 represents a pressure wave that is reflected fromthe location proximate where the PPG sensor 212 is positioned backtoward the heart. Characteristics of the primary peak 504 and/or thetrailing peak 506 may be analyzed by the pulse trending module 249(shown in FIG. 3) to calculate the stroke volume or other physiologicalparameters of the patient.

As shown in FIG. 5, each pulse 502 a-502 n has a particular amplitude.For example, the pulse 502 a has an amplitude A. The amplitudes A maydiffer with respect to one another. In general, the overall amplitude ofthe PPG signal 500 over time t may modulate. The pulse bending module249 (shown in FIG. 3) of the monitor 214 may track and store themagnitude of the amplitude modulation of the PPG signal 500 over time t.Optionally, the pulse trending module 249 of the monitor 214 may trackand store the magnitude of the amplitude of any number of the pulses 502a-502 n for use in determining a stroke volume, cardiac output or otherphysiological parameter of the patient. For example, the pulse trendingmodule 249 of the monitor 214 may use a single pulse 502 a, analyze thesingle pulse to determine a trending nature of the waveform thereof, andcalculate the stroke volume, cardiac output or other physiologicalparameter of the patient based on the single pulse 502 a. Alternatively,the pulse trending module 249 may use multiple pulses 502 a-502 n,analyze the trending nature of the waveforms thereof, and calculate thestroke volume, cardiac output or other physiological parameter of thepatient based upon a comparison of the amplitudes or other aspects ofthe waveforms of the pulses 502 a-502 n to calculate the stroke volume,cardiac output or other physiological parameter of the patient.Optionally, the pulse trending module 249 of the monitor 214 maydetermine an average modulation of the pulses 502 a-502 n over a timeperiod t and use the average modulation to calculate the stroke volume,cardiac output or other physiological parameter of the patient.

As shown in FIG. 5, the frequency of the pulses 502 a-502 n may vary.For example, the frequency U of the pulses over a first period of timemay vary from a frequency f₂ over a later period of time. The monitor214 (shown m FIG. 2) may monitor and determine the frequencies f₁ andf₂. The frequency variation may be based upon respiration, bloodpressure, heart rate, or other factors. The pulse trending module 249 ofthe monitor 214 may detect a magnitude of frequency modulation over atime period t. The pulse bending module 249 of the monitor 214 may usethe frequency of the pulses, or any other temporal element of thepulses, to analyze the trending nature of the waveforms thereof, andcalculate the stroke volume, cardiac output or other physiologicalparameter of the patient.

FIG. 6 illustrates a pulse waveform 600 of a PPG signal, according to anembodiment while the pulse waveform 600 is described as a component of aPPG signal, the pulse waveform 600 may be a component of various otherphysiological signals. The pulse waveform 600 represents a singleheartbeat and may include a primary peak 602 separated from a trailingpeak 604 by a dicrotic notch 606.

The pulse waveform 600 is defined by a plurality of internal pulsecomponents 608 of a given pulse. The internal pulse components 608aggregate together, and may aggregate with internal pulse components ofprevious pulses) to define the pulse waveform 600. The internal pulsecomponents 608 generally include an initial pressure wave 610 generatedfrom the heart to the point of detection, such as in a finger where thePPG sensor 212 (shown in FIG. 2) is positioned, and a series ofreflected pressure waves 612, 614 (any number of reflected waves mayoccur, while only two are illustrated in FIG. 6 as additional reflectedwaves have diminishing effects on the pulse waveform 600). The primarypeak 602 is dominated by the initial pressure wave 610 (but may beaffected by reflected waves of previous pulses as evidenced by theslight increase in amplitude of the primary peak 602 as compared to theinitial pressure wave 610). The trailing peak 604 is defined mostly, ifnot entirely, by the reflected pressure waves 612, 614 that arereflected from the location proximate where the PPG sensor 212 ispositioned back toward the heart.

Various waveform characteristics may be measured and/or calculated fromthe pulse waveform 60Q. The waveform characteristics may be used by thepulse trending module 249 (shown in FIG. 3) to calculate the strokevolume, cardiac output or other physiological parameter of the patient.For example, as described in further detail below, the pulse trendingmodule 249 may utilize a wave period, a pulse period, a transit time, anamplitude, a peak, a temporal element, a gradient, a change in any ofthe waveform characteristics, and the like to calculate the strokevolume, cardiac output or other physiological parameter of the patient.

An internal wave period 616 is a measure of a temporal element of theinitial pressure wave 610. For example, the internal wave period 618 maybe indicative of a transit time of a given pulse. Optionally, thereflected pressure waves 612, 614 may have the same wave periods as theinitial wave period. The wave period 618 may be affected by otherphysiological conditions of the patient, such as blood pressure, heartrate, respiration, and the like. The wave period 616 is variabledepending on the physiological status of the patient.

A pulse transit time (PTT) 618 is a measure of a temporal element of thepulse. For example, the PTT 618 may be a transit time of a given pulsefrom the heart to the location proximate where the PPG sensor 212 ispositioned. The PTT 618 may be calculated by using an ECG system todetect the pulse at the heart and a PPG system to detect the pulse atthe finger, and the time difference between the detection at the heartand the detection at the finger corresponds to the PPT 618. The PTT 618may be calculated by using a dual-pleth system where two PPG sensors areattached at two different locations of the patient and measuring adifferential time between detection of the pulse at the first PPG sensorand detection of the pulse at the second PPG sensor (e.g. at a fingerand at an ear). The time difference between the pulse defectionscorrespond to the PPT 618. Other methods of detecting and/or calculatingthe PPT 618 may be used in other embodiments. The PPT 618 may beaffected by other physiological conditions of the patient, such as bloodpressure, heart rate, respiration, and the like. The PTT 618 is variabledepending on the physiological status of the patient.

A second pulse waveform 620 is illustrated in FIG. 6 and is representedby a dashed line along the waveform. Internal pulse components 622 ofthe second pulse are also illustrated by dashed lines of correspondinginternal pulse components. A pulse period 624, defined by the heart rate(HR) of the patient, may be calculated by measuring the time differencebetween the pulses. For example, the pulse period 624 may be smeasurement of the time difference from the initiation of the pulsewaveform 600 to the initiation of the second pulse waveform 620.Alternatively, the pulse period 624 may be a measurement of the timedifference from a peak 626 of the pulse waveform 600 to a peak 628 ofthe second pulse waveform 620. The pulse period 624 may be affected byother physiological conditions of the patient, such as blood pressure,heart rate, respiration, and the like. The pulse period 624 is variabledepending on the physiological status of the patient.

In an embodiment, characteristics of the pulse waveform 600 may beuseful in determining stroke volume, cardiac output or otherphysiological parameters of the patient. For example, the pulse trendingmodule 249 may use a slope transit time 630 associated with the primarypeak 602 to calculate stroke volume, cardiac output or otherphysiological parameters of the patient. The slope transit time 630 ofthe upslope of the primary peak 602 of the pulse waveform 600corresponds to a gradient 632 of the upslope for a given amplitude 634of the pulse waveform 600. As shown in FIG. 6, because the initialpressure wave 610 dominates the shape of the primary peak 602,particularly the upslope of the primary peak 602, the measured gradient632, amplitude 634 and slope transit time 630 of the primary peak 602can be assumed to be equivalent to a gradient (m), amplitude (A) and aslope transit time (STT) of the initial pressure wave 610.

The gradient m provides a measure of the upslope of the initial pressurewave 610. The gradient M is an amplitude change per unit time and maygenerally be represented by:m=A/STT  Equation (1)

where m is the gradient of the initial pressure wave 610, a is theamplitude of the initial pressure wave 610, and STT is the slope transittime of the initial pressure wave 610.

The inverse of the gradient m is the time change per unit amplitude andmay generally be represented by:k=1/m  Equation (2)

where k is the temporal gradient of the initial pressure wave 610. Thetemporal gradient k is proportional to STT and is not dominated by HReffects, as described in further detail below. Optionally, the temporalgradient k=STT. The temporal gradient k may be used by the pulsetrending module 249 to calculate stroke volume. For example, SV may be afunction of k (e.g., f(k)). The pulse trending module 249 may calculateSV according to the following equations:SV=a(k)+b  Equation (3)SV=K(a(k)+b)  Equation (4)SV=a(ln[k])+b  Equation (5)

where a and b are empirically-determined constants that may bedetermined through clinical examinations of patients, K is a calibrationconstant, and ln is the natural logarithm. The constants a, b and K maybe dependent upon the nature of the subject and/or the nature of thesignal detecting devices. The constants a, b and K may be computed fromrelationships derived from observed historical data (e.g., relationshipswith patient demographic data such as body mass index (BMI), height,weight, and the like) and/or measured signal characteristics (e.g.,heart rate, PTT, differential PTT, and the like). The calibrationconstant K may be derived in whole or in part by calibration throughdilution methods for obtaining stroke volume or cardiac output or othermethods, after which the SV or CO may be calculated continuously usingthe calibrated equation of SV with respect to k.

In some embodiments, the PPG signal may be corrected or normalized toaccount for changes in vascular tone and/or motion artifacts throughanalysis of the PPG signal. Normalizing may be performed prior tocalculating k.

In some embodiments, the pulse trending module 249 may calculate thegradient m, amplitude A and/or slope transit time STT of the pressurewave 610 as a function of the measured gradient 632 and slope transittime 630, such as by predicting the effects of reflected waves ofprevious pulses based on wave periods, pulse periods, pulse transitlimes, shapes of previous waveforms, and the like,

FIG. 7 illustrates a pulse waveform 650 of a PPG signal, according to anembodiment. The pulse waveform 650 is similar to the pulse waveform 600shown in FIG. 8, however the pulse waveform 650 is a pulse waveformtaken during inhalation whereas the pulse waveform 600 is a pulsewaveform taken during exhalation. In FIG. 7, like reference numeralsfrom FIG. 6 are used to identify like components.

Comparison of the waveform characteristics between FIG. 6 and FIG. 7illustrate how the pulse waveforms have two competing temporal elementsthat affect the pulse waveforms. For example, during exhalation (FIG.8), the blood pressure is increasing but the heart rate is decreasing.During inhalation (FIG. 7), the opposite occurs where the blood pressuredecreases and the heart rate increases. Owing exhalation (FIG. 6), asthe blood pressure increases, the internal pulse waves travel faster,thus reducing PTT 618 and having shorter wave periods 616. However, thepulse period 624 lengthens as the heart rate decreases during theexhalation phase of the respiratory cycle. During inhalation (FIG. 7),as the blood pressure decreases, the internal pulse waves travel slower,thus increasing PTT 618 and having longer wave periods 616. However, thepulse period 624 shortens due to the increasing heart rate. The opposingtemporal effects make analysis of the pulse waveforms difficult as thetime measured between two separate fiducial points on a single pleth orpulse waveform may be pulled in competing directions.

As shown in FIGS. 6 and 7, the gradient 632 of the upslope of theprimary peak 602 corresponds to the PTT 618 and wave period 616. As thePTT 618 and wave period 616 increase (e.g. during inhalation), the bendof the gradient is to decrease, whereas as the PTT 618 and wave period618 decrease (e.g. during exhalation), the trend of the gradient is toincrease. However, during both exhalation and inhalation, the gradients632 of the upslopes associated with the primary peaks 602 closely followthe actual gradients m of the initial pressure waves 610. The measuredgradients 632 are a close approximation of the actual gradients m of thepressure wave 610. Such correlation between the measured gradient 632and the actual gradient m occurs because the initial pressure wave 610dominates such temporal region of the resulting (aggregate) plethcardiac pulse waveform. Measurements of the gradients 632 and STTs 630over a fixed amplitude 634 (e.g. height change of the waveform) at ahighly localized region along the upslopes of the primary peaks 602eliminates the effect of the heart rate on the pulse waveforms 600, 660that may influence a transit time calculated from two separated fiducialpoints on the waveform. The pulse trending module 249 (shown in FIG. 3)calculates the time period for the pulse waveform 600, 650 to increase afixed amplitude, and uses such pulse trend to calculate SV, such asaccording to equation 3.

FIG. 8 illustrates a flow chart of a method of determining a strokevolume of a patient, according to an embodiment. The method may beperformed by various systems, such as the system 100 (shown in FIG. 1),the system 210 (shown in FIGS. 2 and 3), or other capable systems. Themethod may include acquiring at 800 PPG signals. The PPG signals may beacquired by securing a PPG sensor to an anatomical portion of thepattern and sensing a physiological characteristic of the patient withthe PPG sensor. While the embodiment of the method described hereinreferences acquiring PPG senate, such as using the PPG sensor 212 (shownm FIG. 2), the method may include acquiring other types of physiologicalsignals, such as ECG signals, PCG signals, and/or ultrasound signalsthat characterize or describe cardiac activity. The physiologicalsignals may be obtained from the individual for at least a designatedperiod of time.

In an embodiment, the PPG signal is analyzed by the pulse trendingmodule 249 (shown in FIG. 3). The pulse trending module 249 analyzes thePPG signal to determine waveform characteristics of the PPG signal, suchas an amplitude of the PPG signal, an amplitude trending component ofthe PPG signal (e.g., a change in amplitude per unit time, a rate ofincrease of the amplitude, a rate of decrease of the amplitude, and thelike), a temporal component of the PPG signal, a temporal trendingcomponent of the PPG signal (e.g., a change in time per unit amplitude),and the like.

At 802, the PPG signal is analyzed to determine a gradient of theupslope of the primary peak of the PPG signal. The gradient maycorrespond to a fixed height change or amplitude of the PPG signal.Then, at 804, the system correlates the gradient of the primary peak toa gradient of the pressure wave. Because the PPG signal is anaggregation of pressure waves and reflected pressure waves, the systemmay determine or calculate the actual gradient of the pressure wave fromthe measured PPG signal. In an embodiment, because the initial pressurewave dominates the measured PPG signal at the upslope of the primarypeak, the measured gradient may be assumed to be equal to the gradientof the pressure wave. In such embodiment, the system uses the measuredgradient as the actual gradient. Alternatively, the system may calculatethe actual gradient from the measured gradient by using a seatingfactor, an algorithm, a look-up table, or by other methods. Optionally,the system may calculate the gradient as a function of the measuredgradient by predicting the effects of reflected waves of previous pulsesbased on wave periods, pulse periods, pulse transit times, shapes ofprevious waveforms, and the like.

Similarly, at 806, the PPG signal is analyzed to determine a slopetransit time (STT) of the upslope of the primary peak of the PPG signal.The STT may correspond to a fixed height change or amplitude of the PPGsignal. Then, at 806, the system correlates the STT of the primary peakto a STT of the pressure wave. Optionally, the system may determine theSTT as an inverse of the gradient. For example, because the gradientm=A/STT, for a unit amplitude the STT=k=1/m.

At 810, a stroke volume is calculated based on the STT of the pressurewave. The stroke volume is calculated as a function of STT. The strokevolume may be calculated according to any of equations 3-6. The strokevolume may be calculated with a pleth-only system. For example, thesystem may be operated without the need for an invasive monitoringsystem, an ECG or any other monitoring system. The system may calculatethe stroke volume with the use of a single PPG sensor. Further, thesystem, at 812, may calculate the cardiac output based on the strokevolume and the heart rate. For example, the CO=SV*HR.

FIG. 9 illustrates a flow chart of a method of operating a PPG system,according to an embodiment. The method may be performed by varioussystems, such as the system 100 (shown in FIG. 1), the system 210 (shownin FIGS. 2 and 3), or other capable systems. The method may includeacquiring at 820 PPG signals. In an embodiment, the PPG signal isanalyzed by the pulse trending module 249 (shown in FIG. 3). The pulsetrending module 249 analyzes the PPG signal to determine waveformcharacteristics of the PPG signal.

At 822, the PPG signal is analyzed to determine an amplitude, a gradientand/or a slope transit time of the upslope of the primary peak of thepulse. Then, at 824, a stroke volume is calculated based on the STT ofthe pulse. The stroke volume is calculated as a function of STT. Thestroke volume may be calculated according to any of equations 3-5.

At 826, the system displays the stroke volume on a monitor, such as themonitor 214 (shown in FIG. 2). The stroke volume may be displayed as anumber, a grade, a graphical representation, and the like.

At 828, the system determines if the stroke volume exceeds a threshold.The threshold may be based on physiological conditions of the patient,such as age, weight, height, diagnosis, medications, treatments, and soforth. If the stroke volume exceeds the threshold, the system at 830provides an alarm condition. The alarm may be a visual alarm, an audiblealarm, or another type of alarm. The alarm may be triggered on themonitor 214 and/or may be transmitted to another location, such as acentral monitoring station to alert medical professionals.

Various embodiments described herein provide a tangible andnon-transitory (for example, not an electric signal) machine-readablemedium or media having instructions recorded thereon for a processor orcomputer to operate a system to perform one or more embodiments ofmethods described herein. The medium or media may be any type of CD-ROM,DVD, floppy disk, hard disk, optical disk, flash RAM drive, or othertype of computer-readable medium or a combination thereof.

The various embodiments and/or components, for example, the controlunits, modules, or components and controllers therein, also may beimplemented as part of one or more computers or processors. The computeror processor may include a computing device, an input device, a displayunit and an interface, for example, for accessing the Internet. Thecomputer or processor may include a microprocessor. The microprocessormay be connected to a communication bus. The computer or processor mayalso include a memory. The memory may include Random Access Memory (RAM)and Read Only Memory (ROM). The computer of processor further mayinclude a storage device, which may be a hard disk drive or a removablestorage drive such as a floppy disk drive, optical disk drive, and thelike. The storage device may also be other similar means for loadingcomputer programs or other instructions into the computer or processor.

As used herein, the term “computer,” “computing system,” or “module” mayinclude any processor-based or microprocessor-based system includingsystems using microcontrollers, reduced instruction set computers(RISC), application specific integrated circuits (ASICs), logiccircuits, and any other circuit or processor capable of executing thefunctions described hereto. The above examples are exemplary only, andare thus not intended to limit to any way the definition and/or meaningof the term “computer” or “computing system”.

The computer, computing system, or processor executes a set ofinstructions that are stored to one or more storage elements, in orderto process input data. The storage elements may also store data or otherinformation as desired or needed. The storage element may be in the formof an information source or a physical memory element within aprocessing machine.

The set of instructions may include various commands that instruct thecomputer or processor as a processing machine to perform specificoperations such as the methods and processes of the various embodimentsof the subject matter described herein. The set of instructions may bein the form of a software program. The software may be to various formssuch as system software or application software. Further, the softwaremay be to the form of a collection of separate programs or modules, aprogram module within a larger program or a portion of a program module.The software also may include modular programming in the form ofobject-oriented programming. The processing of input data by theprocessing machine may be in response to user commands, or in responseto results of previous processing, or to response to a request made byanother processing machine.

As used hereto, the terms “software” and “firmware” are interchangeable,and include any computer program stored in memory for execution by acomputer, including RAM memory, ROM memory, EPROM memory, EE PROMmemory, and non-volatile RAM (NVRAM) memory. The above memory types areexemplary only, and are thus not limiting as to the types of memoryusable for storage of a computer program.

As discussed above, embodiments may provide a system and method ofdetermining a stroke volume of a patient through analysis ofphysiological signals, such as PPG signals, by analyzing waveformcharacteristics of the PPG signal and calculating a slope transit timeof the PPG signal. The slope transit time is less influenced bymomentary rises in the HR and is a good measurement tool for analyzingthe PPG signal to determine stroke volume.

Certain embodiments of the present disclosure may include some, all, ornone of the above advantages. One or more other technical advantages maybe readily apparent to those skilled in the art from the figures,descriptions, and claims included herein. Moreover, while specificadvantages have been enumerated above, various embodiments may includeall, some, or none of the enumerated advantages.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, the above-describedembodiments (and/or aspects thereof) may be used in combination witheach other. In addition, many modifications may be made to adapt aparticular situation or material to the teachings without departing fromits scope. While the dimensions, types of materials, and the likedescribed herein are intended to define the parameters of thedisclosure, they are by no means limiting and are exemplary embodiments.Many other embodiments will be apparent to those of skill in the artupon reviewing the above description. The scope of the disclosureshould, therefore, be determined with reference to the appended claims,along with the full scope of equivalents to which such claims areentitled. In the appended claims, the terms “including” and “in which”are used as the plain-English equivalents of the respective terms“comprising” and “wherein.” Moreover, in the following claims, the terms“first,” “second,” and “third,” etc. are used merely as labels, and arenot intended to impose numerical requirements on their objects. Further,the limitations of the following claims are not written inmeans-plus-function format and are not intended to be interpreted basedon 35 U.S.C. § 112, sixth paragraph, unless and until such claimlimitations expressly use the phrase “means for” followed by a statementof function void of further structure.

What is claimed is:
 1. A photoplethysmogram (PPG) system comprising: aPPG sensor configured to generate a PPG signal including a primary peakseparated from a trailing peak by a dicrotic notch, the PPG sensorcomprising: one or more light sources configured to emit light into atissue of a patient; and one or more detectors configured to detect anamount of the light from the tissue, wherein the detected amount of thelight corresponds to a physiological characteristic of the patient,wherein the PPG sensor comprises a non-invasive PPG sensor; andprocessing circuitry configured to: receive the PPG signal from the PPGsensor, determine a slope transit time of an upslope of the primary peakof the PPG signal, and determine a stroke volume of the patient based onthe determined slope transit time.
 2. The PPG system of claim 1, whereinthe processing circuitry is configured to determine the slope transittime of the upslope of the primary peak of the PPG signal by at least:determining a contour of the PPG signal along the upslope of the primarypeak, and determining the slope transit time based on an amplitude ofthe contour.
 3. The PPG system of claim 1, wherein the processingcircuitry is configured to determine the slope transit time by at leastdetermining a gradient of the upslope for a given amplitude of the PPGsignal.
 4. The PPG system of claim 1, wherein the processing circuitryis configured to determine the slope transit time by at leastdetermining a temporal gradient of the upslope, wherein the processingcircuitry is configured to determine the slope transit time as afunction of the temporal gradient.
 5. The PPG system of claim 1, whereinthe processing circuitry is configured to determine the stroke volumebased on the determined slope transit time using one or more empiricallydetermined constants.
 6. The PPG system of claim 5, wherein the one ormore empirically determined constants comprise a calibration constant.7. The PPG system of claim 1, wherein the primary peak is indicative ofa pressure wave, and wherein the processing circuitry is configured todetermine the stroke volume based on the determined slope transit timeby at least correlating the determined slope transit time to a slopetransit time of another pressure wave.
 8. The PPG system of claim 1,further comprising a display, wherein the processing circuitry isconfigured to present the stroke volume on the display.
 9. The PPGsystem of claim 1, wherein the processing circuitry is furtherconfigured to: determine the stroke volume is greater than a threshold,and trigger an alarm in response to determining the stroke volume isgreater than the threshold.
 10. The PPG system of claim 1, wherein theprocessing circuitry is configured to determine the stroke volume usingsensed signals from only the non-invasive PPG sensor.
 11. A methodcomprising: receiving, by processing circuitry, a PPG signal from a PPGsensor, wherein the PPG sensor comprises a non-invasive PPG sensor;determining, by the processing circuitry, a slope transit time of anupslope of a primary peak of the PPG signal; determining, by theprocessing circuitry, a stroke volume of the patient based on thedetermined slope transit time; presenting, by the processing circuitry,the stroke volume on a display; determining, by the processingcircuitry, that the determine stroke volume is greater than a threshold;and triggering, by the processing circuitry, an alarm in response todetermining that the stroke volume is greater than the threshold. 12.The method of claim 11, wherein determining the slope transit time ofthe upslope of the primary peak of the PPG signal comprises: determininga contour of the PPG signal along the upslope of the primary peak; anddetermining the slope transit time based on an amplitude of the contour.13. The method of claim 11, wherein determining the slope transit timecomprises determining a gradient of the upslope for a given amplitude ofthe PPG signal.
 14. The method of claim 11, wherein determining theslope transit time comprises determining a temporal gradient of theupslope, and wherein determining the slope transit time comprisesdetermining the slope transit time as a function of the temporalgradient.
 15. The method of claim 11, wherein determining the strokevolume based on the determined slope transit time comprises determiningthe stroke volume based on the determined slope transit time and one ormore empirically determined constants.
 16. The method of claim 15,wherein the one or more empirically determined constants comprise acalibration constant.
 17. The method of claim 11, wherein the primarypeak is indicative of a pressure wave, and wherein determining thestroke volume based on the determined slope transit time comprisescorrelating the determined slope transit time to a slope transit time ofanother pressure wave.
 18. The method of claim 11, wherein determiningthe stroke volume is based on sensed signals from only the non-invasivePPG sensor.
 19. A photoplethysmogram (PPG) system comprising: anon-invasive PPG sensor configured to generate a PPG signal including aprimary peak separated from a trailing peak by a dicrotic notch, the PPGsensor comprising: one or more light sources configured to emit lightinto a tissue of a patient; and one or more detectors configured todetect the light emitted from the one or more light sources, wherein thedetected light corresponds to a physiological characteristic of thepatient; and processing circuitry configured to: receive the PPG signalfrom the PPG sensor, determine a contour of the PPG signal along anupslope of the primary peak, determine a slope transit time of theupslope of the primary peak of the PPG signal by at least: determining agradient of the upslope for a given amplitude of the PPG signal, ordetermining a temporal gradient of the upslope, and determine a strokevolume of the patient based on the determined slope transit time,wherein the processing circuitry is configured to determine the strokevolume based on sensed PPG signals from only the non-invasive PPGsensor.
 20. The PPG system of claim 19, further comprising a display,wherein the processing circuitry is further configured to: determine thestroke volume is greater than a threshold, and trigger an alarm on thedisplay in response to determining the stroke volume is greater than thethreshold.